Aerodynamic fairings can make a measurable difference in fuel economy when mounted at the rear of a trailer. But they have to be deployed to make a difference, and be protected from damage.
The Rocketail Wing promises to save 8.42 liters of fuel per 1,000 km by smoothing out the rearward flow of air behind the trailer, while also producing lift like the wing on an aircraft.
The wings sit an inch from the trailer wall to support a uniform air flow, and extend just 14 inches behind the barn door. When the doors are open, these wings also sit flush at the side of the trailer, so the doors can swing a full 270 degrees without being blocked.